Breast cancer is a disease of women due to the actions of estrogens (E) and progesterone (P). While oophorectomy is protective, hormone replacement therapy with E+P increases breast cancer risk compared to E-only. This implicates P, either alone or with E, in tumor promotion. Though many studies address the actions of E or P alone, we contend that one must understand how these two hormones work together. E and P activate estrogen (ER) or progesterone (PR) receptors. Two PR isoforms, PR-A and PR-B, are equal in the normal breast but dysregulated in cancers. Tamoxifen treated patients whose tumors are PR-A rich relapse faster than patients with PR-B rich tumors. Importantly, PR-A and PR-B differentially influence E-dependent tumor growth in either the presence or absence of P. We postulate that PR-A and PR-B in the absence or presence of P, differentially influence effects of E and ER on tumors and that PR-A are especially harmful. AIM 1. Mechanisms of ligand dependent (LD) and ligand independent (LI) transcription by PR.A vs. PR-B. In the presence of P, PR-A and PR-B exert different effects on gene transcription with PR-B more powerful. In the absence of P, PR-A and PR-B are also different with PR-A more powerful. We also show that in addition to Progesterone Response Elements (PRE), LD and LI PR-regulated promoters contain co-Response elements (coRE). This aim dissects the role of PRE and coRE in differential gene regulation by PR-A and PR-B, and addresses mechanisms of LI gene regulation. AIM 2. PR isoforms and in vivo LI and LD effects on E regulated tumor biology and therapeutics. Our preliminary data, using ER+, E-dependent xenografts in ovariectomized non-P supplemented mice, show that presence of PR-A suppresses tumor growth. This aim asks: Why are PR-A+ tumors smaller than PR-B+ ones? How do PR influence tumor responses to antiestrogens? Why do PR block apoptosis by Taxanes? AIM 3. The role of ER and PR and their hormones in breast cancer cell metastasis and growth. Metastasis of ER+, PR+ breast cancers is the major killer of women yet conventional "wisdom" holds that such tumors do not metastasize. Our models show metastasis of ER+, PR+ tumors. This aim develops new models of E-dependent ER+, PR+ metastatic breast cancer and defines the role of steroid receptors and hormones in this process. In sum, we will develop new models to test the hypothesis that PR, even in the absence of P, modify ER+ tumor cell behavior, and that PR-A increase tumor aggressiveness and are harmful.